1. Technical Field
The present invention relates to a circuit device, capable of transmitting an electrical signal between two circuits unconnected to each other, and having high noise resistance.
2. Related Art
To transmit an electrical signal between two circuits that are different in potential of the electrical signal to be input, a photo coupler is generally employed. The photo coupler includes a light emitter such as a light emitting diode and a photodetector such as a photo transistor, and serves to convert the electrical signal that has been input into light with the light emitter, and restore the electrical signal from the light with the photodetector, to thereby transmit the electrical signal.
The photo coupler is, however, difficult to be made smaller because of including the light emitter and the photodetector. Besides, in the case where the electrical signal has a high frequency, the photo coupler is unable to follow up the electrical signal. To overcome such problems, a technique of inductively coupling two inductors thereby transmitting the electrical signal has lately been developed.
Meanwhile, Japanese Published patent application A-H06-120048 discloses a thin-film transformer unit. In the thin-film transformer unit, four thin-film transformers having a primary and a secondary coil are disposed on a surface of one silicon substrate. The adjacent transformers share an outermost peripheral coil. In the four thin-film transformers, the primary coils are mutually connected in parallel, and the secondary coils are also mutually connected in parallel.
In the process of transmitting the electrical signal by inductively coupling two inductors, a noise signal may be generated when an external magnetic field is applied to the inductor of the receiving party. It is, therefore, necessary to suppress the generation of such noise signal because of the external magnetic field.